Optical surface sensors such as those described in U.S. Pat. No. 4,092,068 issued May 30, 1978 to Lucas et al, are particularly adapted for use in the paper industry to determine the surface roughness or surface smoothness of the paper sheet on a continuous basis as the sheet is travelling at relatively high speeds. Such devices are now installed on several paper machines. However, there is no means for checking the calibration of such devices nor is there means for comparing the outputs from such sensors in different mills i.e. thereby provide a more absolute measure of roughness.